In conventional solar power generation systems, the solar cells are connected to a battery to be charged via, for example, a controller circuit including a diode. These systems are generally designed to generate significant charging current even in low radiation conditions, for example, by controlling the number of solar cells used in the solar power systems. In such a conventional system, the power generated by the solar cells must be, for example, three times the power needed to charge the battery. For example, a conventional solar cell, when charged, provides approximately 0.45 volts at its output. A number of solar cells can be joined together in order to provide the desired output voltage. Thus, to power a 10 Watt device, for example, the battery designed for the device would be 20 Watts and the solar cells providing power to the battery would be three times the size of the battery, e.g., 60 Watts. The appropriate number of solar cells would be assembled to provide the 60 Watt output.
The relative size, number and arrangement of the solar cells is selected in order to gain sufficient current in the direction of the battery. The size of the solar cell relates to the current generated by the solar cell while the number and arrangement of the solar cells relate to the voltage of the solar power system. While the size of the solar cells can be increased to get the desired current, the cost of solar cells are relative to the size. Therefore, for solar cells of moderate size and/or designed for limited space applications (e.g., a small lighting device used as lane markers on a highway), if there is low radiation on the solar cells, e.g., a cloudy day, then an insufficient amount of current is generated by the solar cells to provide charge in the direction of the battery.
Solar cells achieve maximum voltage even in a low radiation condition when in an open circuit condition, e.g., when not connected to a battery to be charged, with each standard solar cell providing an approximately 0.45 volt output. In a conventional system, however, where the solar cells are connected to the battery via a diode connection, the voltage generated by the solar cells when there are low levels of radiation on the solar cells falls immediately from the voltage of the solar cells (e.g., Vcc) to the voltage of the battery. As an example, if twelve solar cells were provided to charge a 2 volt battery, the solar cells would output approximately 6 V (12.times.0.45 V), which is approximately three times the voltage of the battery. In a low radiation situation, the output of the solar cells is connected to the battery and the voltage of the output will fall from approximately 6 V to the voltage of the battery, e.g., 2 V. After the voltage of the solar cells falls to the voltage of the battery, an equilibrium situation arises with the voltage of the solar cells equaling that of the battery plus the voltage drop over the diode. After this time, the equilibrium situation results in effectively no current being generated in the direction of the battery. For example, using the diode connection, only one spark is generated, for the duration of the time of the voltage to fall from Vcc to Vbat, thus limiting the charging current to the battery, and then the circuit remains in the equilibrium condition with no further charging of the battery and no means for disconnecting the solar cells to regain their maximum voltage. Accordingly, in order to generate enough current to charge the battery, the number of the solar cells must be increased, thereby increasing costs, complexity, space requirements, etc.
In addition, in a solar power system designed for a low radiation situation that is then exposed to high radiation levels, the conventional diode connection between the solar cells and the battery to be charged provides a charging voltage, which is higher than the voltage of the battery, at a high current which can damage the battery to be charged. Even in a conventional solar power system designed to provide a charging voltage at low levels of radiation that is within 10% of the voltage of the battery, for example to charge a Nickel-Cadmium battery, such a system would provide too much charging current in a high radiation situation. For example, the high charging current resulting from a high level of radiation incident on the solar cells can heat the battery junction and shorten the life of and possibly ruin the battery. In addition, the high charging current is a constant current that can also be harmful to the battery, particularly when the battery has reached its capacity and the charging current is still being provided. Thus, prior art solar power systems required a design for either low or high levels of incident radiation, but a single such system will not work in both types of radiation situations.